1. Field of the Invention
The field of invention relates to a device and method for using a rotary tractor in a horizontal well bore.
2. Description of the Related Art
In horizontal drilling, there are many challenges to maintaining operations that are not present in vertical or even deviated systems. Gravity pulls the metal drill pipes, the drill collars, the drill bit and downhole tools against the walls of the well bore. The frictional force generated both while moving and when idle can damage the equipment. Much more energy is required to move a similar distance from the surface entry point horizontally than vertically. Today, extended reach completions reaching 10,000 to 12,000 horizontal feet in unlined or minimally lined well bores exist. Longer distances are envisioned.
During the running of intelligent completion systems, hydraulic or electrical lines, or both, are positioned on the exterior of piping or tubing. The control and electrical lines permit the operation of mechanical sleeves and equipment in the downhole environment as well as provide a conduit for transferring data and commands. Introducing these systems into a horizontal well having a long horizontal section, including extended reach wells (ERWs), multi-lateral and multi-tier wells and exposes operational difficulties. These systems by themselves have difficulty reaching the technical objective (that is, the end of the well bore or Total Depth) due to the effect of gravity and friction.
Overcoming the friction of the horizontal section of the horizontal well bore is a significant, problem. Sometimes rotating the drill string temporarily overcomes by transferring the axial friction vector into a rotational vector. However, this is not recommended with intelligent systems because rotating the drill string can damage the external control and electrical lines and cause the completion to fail. Since an intelligent completion string cannot be rotated, the exposure to friction increases with the length of the horizontal section. Lubricants in the wellbore can reduce some effects of friction; however, their use can add complexity in terms of reservoir damage and cleanup. Lubricants are costly and only marginally reduce friction (±10 to 20%). Centralizers can help to overcome friction while running into wells. Centralizers can be made from composites that have a lower coefficient of friction than the drill string and assembly. Centralizers, however, have to be placed regularly along the length of the well bore and are prone to breaking and being pulled apart on the unlined well bore wall.